Welcome to the forum. Some useful information in this link;
The schematic shows individual trim pots for each head. With these old 'skeleton' pots they can sometimes seize and forcing them can cause them to break up. I usually give them a squirt of Deoxit and leave them for a few minutes before adjustment. It's best to read up and get as much information as possible before diving in - it's easy to get old tape machines badly out of shape by making a mistake.